Public-address circuits



Feb., 26, E924@ v 11,4%,983

H. s. HAMMLTQN PUBLIC ADDRESS -CIRCUTS Filed Feb. 9. 1923 2 Sheets-Sheet l X M N Y @2 I MW fwn/2L ATTORNEY PUBLIC ADDRESS CIRCUITS Filed Feb. 1923 2 sheets-sheet 2 HO'O'OH ATTORNEY Fatente Feb. 26, T94.

HAROLD S. HILTON, on N AND TELEGf s, (wmp i,

reame-ann 1 N. Y., AGNQR T0 ERICAN' y'.llIELEIEHORIEZ A. CORPOTON 0F YRK.

CIRCUITS.

Application led February 9, 1923. serial Re. @13,072.

To aZZ whom it may concern Be it known that LHAROLD S. HAMILTON, residing at New York, in the county of Bronx and State of New York, have invented certain Improvements in Public-Address Circuits, of which the following is a specification.

This invention relates to electrical means for transmitting addresses, music and the like to public assemblages, and more particularly to arrangements whereby a socalled ublic address system may loe associated with long distance telephone lines, so that s eech, music or the like may be transmitte from one point to an audience at a distant point. f

Where a public address system is to be associated with a long distance line so that an audience may be addressed from a distant point, it is usually necessary to provide some arrangement whereby a local audience may be addressed at the same time, and quite frequently it is desirable that each of two speakers at two distant points may be able to address not onl a local audience but an audience at the ot er point. Tt is one of the principal objects of the invention to provide circuit arrangements whereby the above results may be accomplished in a simple, economical and satisfactory manner.

The invention may now be more 'fully understood by the followinv description when read in connection witlli the accompanying drawing in which Figures l, 2 and 3 illustrate three simplified circuity diagrams embodying the principles of the invention while Figs. 4, 5 and 6 are detailed circuits showing portions of the apparatus involved.

Referring to Fig. l, two distant stations X and Y are shown, at each of which is arranged apparatus to be used in addressing an audience. This apparatus is illustrated conventionally and comprises a hi h quality transmitter of a t pe well known in the art which picks up the music or speech to loe transmitted and impresses it on an amplifier A1. Upon the output side of the amplifier a volume indicator V, is provided so that the attendant may determine the amplitude of the currents which are to be transmitted.

This volume indicator may be, for example,

side of the amplifier is connected to a line TLE, which may, for example, be a non-loaded cable leading to a long distance toll office M. The receiving portion ofthe apparatus comprises an am lifier B, associated with a recelvln line R W which may also be of non-loa ed cable. The line RLW leads from the toll office M to the local station X. The output side of the amplifier l5, is connected to loud speaking ap aratus which may comrise one or more oud speakers of a well known type. The loud speaking a paratus is indicated conventionally at R1. volume indicator Vs similar to that already described is bridgjed across the output side ofthe amn plifier 1 for the purpose of indicating to the attendant the volume of the transmission to the loud speaker.

At the toll ofiiceM the transmitting line TLB and receiving line RLW are associated with the toll line ML which interconnects the toll offices M and N through a hybrid coil connection 10, and the toll line ML is balanced by an artificial line or network N1 to render the lines TLE and RLW conjugate. At the toll office N an attenuation equalizer E, is provided in the line TLE for the purpose of compensating for the distortion due to the fact that the non-loaded cable attenuates the hi her frequencies much more than the lower requencies. This attenuation equalizer may be any one of a number of known types, such, for example, as the attenuation equalizer illustrated in application of R. S. Hoyt, Serial No. 242,- 567, filed June 29, 1918, now Patent No. 1,453,980, dated May 1, 1923. One form of the equalizer whose principle oi operation is fully described in the Hoyt application referred to is illustrated in Fig. 6 and consists of a serial combination of inductance and capacity in parallel with a resistance, the elements being so pro ortioned as to introduce distortion comp ementary to that due to the line. The amplifier C, is also provided at the toll office for amplifying the currents to the proper volume for transmis- @lli sion over the toll lme. This amplifier may be of any well known ty e such, for example, as a vacuum tube amp ifier.

The receiving circuit RLW at the toll ofce M includes a receiving amplifier C2 which may be similar to the amplifier 0,. The line RLW also includes an equalizer E2 sililar to the .equalizer E1 already described an having a similar function, viz, to comensate for the distortion introduced by the line RLW. The toll line ML interconnecting the toll offices M and N may be of any known type, and when the toll line is of sufficient length repeaters will, of course, be provided, although as these repeaters form no part of the resent invention they are not illustrateg. The apparatus at the toll office N and station Y is similar to that described at toll oflice M and station X respectively and need not be further described.

Returning to the station X, a by-pass connection F1 is provided between the output of the amplifier A1 included in the transmitting circuit, and the input of the amplifier B1v included in the receiving circuit, in order to enable a speaker Yat the transmitter T1 to address hisflocal audience listeningto the loud 'speaker R1. A similar by-pass circuit F2 is also provided at station Y. If a by-pass such as F1 were not provided, transmission from the transmitter T1 to the local loud speaker R1 could only take place due to the unbalance of the hybrid coil 10, which would be a very unsatisfactory arrangement. l

Upon the other hand, the provision of a two-way connection as F1 between the circuits TLE and RLW would enable the energy transmitted over ythe outgoing circuit TLE to be transmitted through the hybrid coil 10 to the incoming circuit BLW, duel to unbalance of the hybrid coil, and from the incoming circuit RLW the energy would be transmitted directly to the outgoing circuit TLE so that singing might be set up. It is therefore desirable that the by-pass circuit F1 be a one-way circuit, s0 that transmission can onl take place from the output of the ampli 1er A1 to Vthe input of the amplifier B1, while no transmission takes place in the reverse direction.

Fig. 4 illustrates a detailed circuit arrangement for accomplishing this result. In this figure the amplifier A1 is shown as a vacuum tube amplifier of known type, and a potentiometer 12 is shown associated with the input circuit of the amplifier for cointrolling its gain. The amplifier B1 is likewise shown as a vacuum tube amplifier, and a potentiometer 13 is associated with its input circuit for adjusting the gain. The by-pass circuit F1 is provided with an amplifier A of the vacuum tube type. As is well kno-wn, an amplifier of this type will transmit in one direction only, and no current can be transmitted from the circuit RL through the amplifier A to the circuit TL. c

A modified arrangement for accomplishing the same result is shown in Fig. 5. Here the amplifier A is omitted and the by-pass F1 includes merely ha transformer Lacasse for coupling the b-pass circuit to the input of the amplifier 1. If desired, a potentiometer 14.- may be included upon the4 secondary side of the transformer. Owing to the lfact that the grid of the amplifier B1 is made ultranegative with respect to the filament by means of the C battery, no current-can flow in the input circuit of the amplifier B1, and the only effect of currents impressed upon said input circuit through the input transformer or the transformer of the by-pass F1 is to produce potential variations in the input circuit without producing any corresponding current flow. The result is that current flowing in the primary winding of the transformer of the by-pass F1 produces potential variations in the input circuit of the amplifier B1 therebyl causing the amplifier to produce corresponding currents in its output circuit Ito actuate the loud speaker R1. Upon the other hand, currents incoming from the line RL and producing only potential variations without corresponding current flow in the input circuit of the amplifier B1 do not induce any currents in the upper or primary winding of the transformer of the by-pass F1, because only potential variations are produced in the lower winding, and no current flows to induce current in the upper winding. Consequently, transmission can take place in one direction only.

The provision of the volume indicator V1 upon the output sideof the amplifier A1 is an important feature of the circuit, as it enables proper adjustment of the potentiometer upon the input side of the amplifier A1 to insure that the power delivered t0 the toll line ML will be within proper limits. If the volume at the toll office is allowed to become too great, the telephone repeaters (if any) on the toll line will be overloaded and serious distortion will result. .Upon the other hand, if the volume is allowed to become too weak, extraneous noise and cross-talk upon the toll line will tend to` obliterate the direct transmission.

`In adjusting the gain of the amplifier A1 spect to the line RL so as to deliver proper volume for the loud speaker R1 both for the local talking circuit and for the reception of addresses from the distant end of the circuit. The volume indicators V1, `and with one-way repeaters D D2, etc.

eficaces V4 at the station Y have functions similar to those of the devices V1 and V8 respectively.

It will be seen that the circuit above described interconnects the toll line with the transmitter and loud speaker of the public address system through a four-wire circuit. lt will also be apparent that a speaker at transmitter T, may simultaneously address audiences at X and l through the projector apparatus R, and R2 respectively. Likewise a speaker using the transmitter T2 may simultaneously address both audiences.

F ig. 2 illustrates a circuit arrangement similar to that of Fig. l except that the toll circuit connectin the toll ofiices M and N is of the four-wire type. A two-wire toll line MEE interconnects the toll oiices M and N for transmission from west to east. This toll line is provided at suitable poSints 1milarly a two-wire toll line MLW for transmitting from east to west interconnects the toll oiiices M and N and is provided with one-way ampliers D, and D2', etc. The local lines TLE and BLW', instead of being interconnected to the toll line through' a hybrid coil, are each separately connected to corresponding lines of the four-wire circuit, the line TLE being connected through the apparatus at the toll ohce to the toll line MLE, and the receiving line RLW being connected through the toll oilice to the toll line MLW. The `connections at the station N are similar to those above described. 'li`he operation of the circuit will be apparent from the operation of the circuit of Fig. 1 and need not be :further described.

Fig. 3 illustrates an arrangement whereby public address apparatus may be associated with the toll line at some intermediate point. in F ig. 3, L and L indicate sections of a toll line connecting distance toli offices which may be associated with public address systems at each end in the manner described in connection with Fig. 1. lin order to associate the public address apparatus with the toll line at an intermediate point, a hybrid coil 15 is provided whereby local circuits TL and RL leading from the toll oiiice to the auditorium are interconnected with the toll line. The apparatus associated wits the local circuits RL and TL at the toll oi'iice will be similar to that associated with the circuits TLE and BLW in Fig. 1. The apparatus located at the auditorium will also be identical with that described in connection with Fig. 1, and description thereof would be superfluous. lt should be noted that the hybrid coil 15 may be so designed that the impedance which it introduces into the toll circuit is so small as to avoid any serious irregularity. The talking currents from the local line TL pass to the line ,sections L and L' through the hybrid coil. The received transmission from the toll line is obtainedI through a high impedance brid e across the midpoints of two of the win ings of the three-winding hybrid coil 15. The amplifiers C, and C2 introduce sufficient gain to overcomethe loss due to the inei'icient coupling of the lephone line.

It will be obvious that the general principles herein disclosed may be embodied in many other organizations widely different from those illustrated without departin from the spirit of the invention as defined in the following claims.

What is claimed is:

1. A public address system for the two-4 way interconnection of distant auditoriums comprising a toll circuit interconnecting two distant toll oilices, a four-wire circuit from each toll oihce to the associated auditorium, a transmitter connected to one iine of the tour-wire circuit, a loud speaking receiver connected to the other line of the four-wire circuit, and a by-pass circuit between the two lines of the four-wire circuit for enabling the transmitter to actuatc the local loud speaker.

2. A public address system for the twoway interconnection of distant auditoriums comprising a toll circuit interconnecting two distant toll oiices, a four-Wire circuit :from each toll office to the associated auditorium, a transmitter connected to one line or the tour-Wire circuit, a loud speaking receiver connected to the other line of the four-wire circuit, and a by-pass circuit between the two lines ot the four-wire circuit for enablingthe transmitter to actuate the local loud speaker, said by-pass transmitting in one direction only.

3. A public address system for the twoway interconnection of distant auditoriums comprising a toll circuit interconnecting two distant toll offices, a four-wire circuit from each toll oiice to the associated auditorium, a transmitter connected to one line of the four-wire circuit, a loud speaking receiver connected to the other line of the fourwire circuit and a by-pass circuit between the two lines of the four-wire circuit tor enabling the transmitter to actuate the local loud speaker, said by-pass including a oneway device for preventing transmission from the receiving circuit to the transmitting circuit.

e. A telephone system comprising a telephone circuit interconnecting two oices, four-wire circuits interconnecting the oces with local stations, a hybrid coil at the oiiices for associating the four-wire circuit with the telephone circuit, a transmitter connected to one line of each four-wire circuit, a receiver connected to the other line of each tour-wire circuit, and a ley-pass connection frmone line of each tour-wire circuit to the other for enabling the transmitter' to actuate the local receiver.

5. 'A telephone system comprising a tele- .phone circuit interconnecting two oces,

four-Wire circuits interconnecting the omces with local stations, a hybrid coil at the oices for associating the four-wire circuit With the telephone circuit, a transmitter connected to one line of each four-Wire circuit, a receiver connected to the other line o each four-wire circuit, and a by-pass connection from one line of each four-Wire circuit to the other for enabling the transmitter to actuatethe local receiver, said bypas circuit transmitting in one direction on 6? A telephone system comprising a telehone circuit interconnecting two smoes; our-wire circuits interconnecting the oces with localstations, a hybrid coil at the ofiices for associating the four-wire circuit with the telephone circuit, a transmitter connected to one line of each fourwire circuit a receiver connected to the other line ci each four-wire circuit, land a by-pass connection from one line of cach four-Wire circuit to the other for enabling the transmitter to actuate the local receiver, said by-pass circuit having a one-way device for preventing transmission from the receiver circuit to the transmitter circuit.

in testimony whereof, )I have signed my name to this specification this 8th day of February, 1923.

. HARLD S. HAli/HLTN. 

